1. Field of the Invention
This invention relates to spray head apparatus and more particularly to spray head apparatus utilizing a pressurized propellant.
2. Description of the Prior Art
The use of pressurized propellants in spray systems to atomize the the material being sprayed is well known to those skilled in the art. It has been found that high pressure propellant systems are not suitable for certain applications such as where, for example, it is desired to spray a uniform and/or very thin layer of the material. This is mainly because a high pressure system tends to blow away the material in the layer as it is being formed by the spray. Accordingly, it is the custom in these type applications to use a low pressure propellant system to minimize or mitigate the deleterious effects of a high pressure propellant system.
For example, in the production of photoresist masks used in the manufacture of printed circuits and/or integrated circuits, the mask is made by initially depositing a continuous photoresist layer on the surface of the workpiece intended to be worked with the mask when the mask is subsequently finalized. The mask is then produced from the layer using well known photolithographical techniques. One known way of depositing the layer in the prior art is to spray on the resist. The spray is formed from a discharging stream of liquid photoresist that is atomized by a low pressure propellant system. If the resist is sprayed on with a high pressure propellant, discontinuities in the resist layer occur as the result of being blown away by the propellant as the layer is being deposited. Consequently, the continuity of the resist layer and hence the integrity of the mask subsequently formed therefrom and/or of the resultant circuitry produced with the mask are adversely effected. The use of a low pressure propellant is thus more suitable for such an application.
Heretofore, in a known spray head apparatus of the prior art, a stream of photoresist fluid is discharged from the unobstructed orifice of a nozzle. On exiting from the orifice, the stream is intercepted by a low pressure propellant, e.g. filtered nitrogen gas, and the resultant turbulence atomizes the photoresist thereby forming the spray. However, it was found that the turbulence was less effective in atomizing the resist at the core or center of the stream than at the periphery of the stream. Thus, within the zone or region formed by the spray, the resist tended to be more thickly deposited at the center of the spray zone than at the periphery. Hence, the prior art apparatus was not conducive to forming a layer of substantially uniform thickness. The problem is even more acute where the thickness of the deposited resist layer approaches the range of forty micro-inches or less. As is well known to those familiar with the art, non-uniformities in the resist layer adversely affects the electrical characteristics of the resultant circuit elements produced with the subsequently formed mask. For example, it can be readily appreciated by those skilled in the art that if the mask is to be used to etch a metallization layer which is eighty thousand angstrom thick into a conductor pattern of plural one mil wide conductors with a minimum spacing of three-tenths of a mil between conductors, a resist layer of non-uniform thickness can result in such adverse characteristics as open or short circuited conductors, and/or non-uniform impedance characteristics of the conductor lines, etc.
Moreover, the orifice of the nozzle of the aforedescribed prior art spray head apparatus was susceptible to clogging which caused diversion of the stream from its designed, i.e. intended, direction and/or further adversely affected the atomization of the stream. As a result, the direction of the spray was also diverted and consequently the spray did not intercept the member being sprayed at the desired location coordinates.
Hence, the aforedescribed prior art spray head apparatus was not readily controllable nor conducive to spraying a resist layer with a reliable uniform thickness, and/or adversely affected the reliability of the subsequently formed therefrom photomask and/or the circuitry thereafter produced from the mask.
It should be understood that in the past elongated pin-like members have been associated with atomizing nozzles and spray devices, cf. U.S. Pat. Nos. 1,812,234 and 2,612,408, and United Kingdom Pat. No. 26,575, A.D. 1912, for example. However, of the prior art of which we are aware, none provide the structural means for discharging the material to be sprayed as a hollow-shaped stream and/or in combination with a pressurized propellant system in accordance with the principles of the present invention as hereinafter described.